This invention relates to a method and apparatus for spotting chromatographic elements with a sample in a reproducible manner.
There is described in a patent application entitled "Automatic Thin Layer Chromatographic Apparatus" filed Jan. 11, 1973, Ser. No. 322,869 by Gene E. Lightner an apparatus for sequentially withdrawing an aliquot of a sample fluid to be analyzed from a supply magazine and transferring such sample aliquot to a chromatographic plate. The automatic apparatus described utilizes two pivotable arms, one a spotter arm, the second pick-up arm. The pick-up arm operates to pick-up chromatographic plates from a supply magazine and place them in a position for spotting. After being spotted with an aliquot of a sample fluid to be analyzed, the plates are transferred by the pick-up arm into discrete vertically disposed tanks positioned about a turntable. Solvent is applied to each tank immediately prior to insertion of the spotted plate therein. Upon development of a plate by the solvent, usually one complete rotation of the tank turntable, and after the solvent is drained from the tank, the pick-up arm picks up the developed plate and passes it down a chute to an automatic reader. The spotter arm has a capillary tube mounted on its end portions. This tube is dipped into the sample vials, which are disposed about a sample turntable in a ring-like fashion, and transfers a sample aliquot to a precise position on the lower edge of a chromatographic plate. A reference sample aliquot may be placed directly over the spot to provide an internal standard.
As is described in the said Lightner application, the capillary tube used for spotting is acutally brought into contact with the plate and raised and lowered several times to insure that all of the sample contained in the tube is transferred to the plate. The capillary action of the interstitial structure of the plate's coating is used to suck the sample from the capillary tube. While satisfactory, it has been found that in some instances acutal contact with the plate tends to disturb the coating material. This can in some cases effect the reeproducibility of the sample. In some cases the spot is sometimes caused to spread unnecessarily in the spotting region.
An even greater problem is the lack of reproducibility with all plate coatings. Some coatings do not provide a reproducible capillary flow rate. The quantity of fluid remaining on the inner walls of the tube will vary according to the flow rate at which the tube is exhausted, the solvents used and the characteristics of the particular capillary tube. Thus with varying flow rates caused by vagaries of the coatings, the liquid remaining on the capillary walls of the tube can and does vary from sample to sample and plate to plate. Naturally, this affects the reproducibility of the analysis.
Accordingly, it is an object of this invention to obviate many of the disadvantages incumbent in spottiing procedures often used in the prior art.
Another object of this invention is to provide an improved apparatus for reproducibly spotting chromatographic plates.
A further object of this invention is to provide an improved apparatus for spotting plates utilizing capillary tubes with positive pressure being used to control sample volume and, if desired, discharge rate of the sample fluid.
Another object of this invention is to provide an improved method of reproducibly spotting chromatographic plates.